The present invention relates to an imaging method using nuclear magnetic resonance (hereafter referred to as NMR), and in particular to an imaging method for deriving a plurality of magnetization distribution images separated for respective chemical shifts.
In an imaging apparatus using the nuclear magnetic resonance phenomenon, it is necessary to separate and discriminate signals coming from an object for inspection in correspondence to respective positions. In one of such methods, a field gradient (a phase encode gradient) having a programmable time intensity product in the first direction is applied to a static magnetic field during a period lasting from excitation of nuclear spin in the object until readout of the resonance signal, and the position information of the above described direction is encoded into a spin phase. The resonance frequency is changed along a second direction by reading out the spin signal under the state that a field gradient (readout gradient) in the second direction is applied to the static magnetic field. The basic principle of this method is described in Journal of Magnetic Resonance, Vol. 18 (1975), pp. 69 to 83 and Physics in Medicine & Biology, Vol. 25 (1980), pp. 751 to 756.
As an evolution of such imaging methods, a method called chemical shift imaging is known. Since respective spins are sensitive to different magnetic fields because of difference in molecular structures around them even though they belong to an identical nuclei, resonance frequencies of respective spins change depending upon positions on the molecular structure. This phenomenon is called by the name of chemical shift.
As a method of separating spins corresponding to respective chemical shifts and imaging respective spin distributions, there is a method called Dixon's method as described in Radiology, Vol. 153 (1984), pp. 189 to 194. In this method, the sum and difference of two images obtained by respective measurement sequences which are different in phase rotation effect caused by chemical shift are calculated, and two chemical shift images (mainly corresponding to water and fat) possessed by hydrogen atoms are separated.
In actual NMR imaging apparatus, the distribution of resonance frequency caused by the intensity distribution of the static magnetic field with respect to position can not be neglected as compared with the difference in resonance frequency caused by the chemical shift even if the magnet for generating the static magnetic field is carefully produced. In the method of Yeung described in Radiology, Vol. 159 (1986), pp. 783 to 786, the above described Dixson's method is improved, and the influence of intensity distribution of a static magnetic field is removed by using the second spin echo. In the Yeung's method as well, however, images with water and fat separated are derived from the sum and difference of two images. Therefore, it is not possible to obtain two desired chemical shift image each of which has n.sup.2 pixels until imaging using a measuring sequence including excitation of spins performed n times is repeated twice. Accordingly, movement of the object in a period during which the measuring sequence is repeated twice causes a large artifact.